Some issued vests are torture to wear, and I can understand a street patrol officer wanting to opt-out. The last vest issued to me, even though I was measured by a rep from the vendor*, was such a torture device, interfering with my breathing, and interfering with my arm movements. When I handed it back in, at retirement, I had worn it for less than an hour, because that is how long I could tolerate it, before I pulled it out from under my uniform shirt, and went without armor for the rest of the shift. The next night, and for a while, I wore a slightly too-big vest I had previously bought from a local ex-deputy, in near-new condition.
My next step was to measure myself, and buy an off-the-shelf, new-old-stock “expired” vest from a reputable police supply place. They had an unwanted Level IIIA that was perfectly comfortable, and would stop more than the Level II issued vest. I knew that unworn Kevlar does not decay, so did not worry about protection. Notably, the vest I bought, for myself, was the same brand as the ill-fitting vest I had been issued.
I also decided to start researching plate carriers, and so ended my career wearing multi-curved Level III plates, with Level IIIA backers and side panels, when on patrol. I would wear the concealed IIIA vest when in uniform, while going to court, the academy, etc.